This invention relates to extrusion dies for apparata producing seamles thermoplastic tubing, particularly double-walled pipes with a smooth inner wall and a corrugated outer wall.
Known dies of such a type (for example U.S. Pat. No. 4,510,013) include the following components:
an extrusion head having a central bore with at least one opening for receiving an extrudate of a thermoplastic material under pressure therefrom; PA1 a nozzle attached to said head and carrying an outer die lip on a free end of the nozzle; PA1 a hollow mandrel placed in said bore in a substantially coaxial relation with and spaced from the nozzle to provide a thermoplastic conducting passage extending in the longitudinal direction thereof for the delivery of the extrudate to the delivery end thereof carrying an inner die lip defining with the outer die lip the annular extrusion orifice of diminishing interior dimensions, through which the extrudate can flow uninterruptedly and form the tubing; PA1 an additional mandrel mounted coaxially with the hollow mandrel, downstream thereof, for urging the tubing outwardly and extracting heat from the latter by means of a plurality of internal channels cooling an outer surface of the additional mandrel receiving the tubing from the extrusion orifice, said channels being connected to a coolant discharge and a coolant supply located exteriorly of the tubing and communicating with said channels through the hollow mandrel. PA1 The apparatus of the above patent works in a blow-molding mode, pressurized air being supplied within the outer tube between the nozzle and the additional mandrel for conforming the tube to a travelling molding cavity, and within the inner tube between the additional mandrel and downstream baffles. PA1 This leads to the following drawbacks: PA1 fusing still unstable and soft inner parison to the already formed outer tube wherein the inner tube turns from a soft to a rigid state; PA1 retraction of the parison from the outer surface of the additional mandrel and drawing the same into the formed corrugations of the outer tube; PA1 nonuniform change of the internal pressure of the inner parison. PA1 said plurality of grooves on the outer lateral surface of the additional mandrel can have at least one annular distributing groove connected to said vacuum source, and a number of helical grooves communicating with said annular groove; PA1 said plurality can have at least two annular distributing grooves, one of which is axially most remote from and one of which is axially most close to the extrusion orifice, said annular grooves being connected to said vacuum source independently, and can have at least two groups of helical grooves communicating respectively with said annular grooves in such a manner that the groups are communicated with said vacuum source independently to ensure possibility of varying the intensity of vacuum pressure along the additional mandrel.
These drawbacks cause imperfections of the inner wall, such as shrink marks (depressions of its surface) in the spots of retractions, warpage (caused by said change) and wrinkles having appearance of a wave.
In another known apparatus (for example, U.S. Pat. No. 3,976,414, FIG. 2) working in a vacuum-molding mode with negative pressure applied exteriorly to the outer tube to draw the latter into a travelling molding cavity, the inner tube is under positive internal air pressure. Herein since the additional mandrel is not cooled, the above imperfections are more apparent.